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Regulators on Wednesday imposed new restrictions on a vast market that played a significant role in the 2008 financial crisis.

The Securities and Exchange Commission voted 3 to 2 to adopt a set of new rules for money market funds, a $2.6 trillion industry where ordinary individuals and sophisticated institutions alike park their money. The rules come after years of debate among regulators and lobbying from Wall Street.

The rules “will reduce the risk of runs in money market funds and provide important new tools that will help further protect investors and the financial system,” Mary Jo White, the S.E.C. chairwoman, said in a statement. “Together, this strong reform package will make our markets more resilient and enhance transparency and fairness of these products for America’s investors.”

The split on the commission reflected the lingering frustrations still felt by many in the debate about money market funds. These funds, which are big lenders to financial institutions, are deeply embedded in the financial system. They were long perceived as virtually risk-free, and many investors continue to treat their shares as if they were cash.

But the risks were vividly on display in the financial crisis, when in the wake of the collapse of Lehman Brothers, one fund, the Reserve Primary Fund, startled investors by “breaking the buck,” or reporting a net asset value below $1 a share. As investors dumped their shares, panic spread throughout the financial system and worsened the gathering crisis. Regulators vowed to find a solution.

The rules approved on Wednesday aim to prevent any future runs through a combination of measures. In one important change, certain money market funds will have to report a floating net asset value instead of a fixed value of $1 a share. This change is meant to remind investors that the funds are not without risk and that their value can decline periodically.

But not all funds will be covered by that rule. Only funds whose investors are institutions and that purchase corporate debt or municipal securities are covered. Funds whose investors are individuals are not subject to the change.

In addition, the S.E.C. adopted rules that give funds the ability to stem investor redemptions during times of stress. Money market funds, in these situations, will be able to impose fees and delays that temporarily prevent investors from taking out their cash.

Wall Street seemed largely pleased with the final result.

The rules were a significant shift from a proposal in 2012 by the previous S.E.C. chairwoman, Mary L. Schapiro, who wanted all funds to adopt the floating net asset value or else hold capital to absorb losses. Ms. Schapiro was unable to garner enough support at the S.E.C. for that proposal.

As the agency drafted a new proposal, one important source of input was the brokerage firm Charles Schwab, which proposed limiting the floating share price rule to certain funds. In a statement on Wednesday, Schwab applauded the S.E.C. and said the rule would “strengthen investor confidence.”

Another big player in money market funds, BlackRock, described the S.E.C.’s process as “thoughtful, deliberate and consultative.” The firm said it was “well positioned” to offer products complying with the rules.

The industry’s main lobbying group, the Investment Company Institute, expressed qualified support. “While we may question some aspects of the rule as adopted, we strongly believe that the S.E.C. has the long regulatory experience and deep technical expertise required to strike the proper balance,” the chief executive, Paul Schott Stevens, said in a statement.

But several important aspects of the rules drew criticism from consumer advocates and two of the S.E.C.’s commissioners, who argued that the risks would not be eliminated.

The delays to withdrawals that funds will be able to impose during crises could introduce new problems, said Kara M. Stein, an S.E.C. commissioner. At the agency’s meeting on Wednesday, she argued that sophisticated investors would be able to sense trouble brewing and move to withdraw their money before the delays are imposed. Investors, she said, would have “a strong incentive to rush to redeem ahead of others,” creating a run on the fund and potentially on other funds as well.

“Ultimately, this contagion could freeze the wholesale funding markets in much the same way as occurred during the recent financial crisis,” said Ms. Stein, who voted against the rules.

The limited scope of the rules was a disappointment to Dennis M. Kelleher, the president of Better Markets, a nonprofit group that pushes for tighter regulations of Wall Street. He said that all money market funds should have been required to adopt a floating net asset value.

“The fiction of a stable N.A.V., and therefore the belief in guaranteed returns, continues for a majority of money market funds,” Mr. Kelleher said.

Reflecting the complexity of the debate, Michael S. Piwowar, another S.E.C. commissioner, attacked that rule for entirely different reasons. Mr. Piwowar, who cast the other dissenting vote, argued that the floating net asset value would impose costs on funds and their shareholders, potentially leading the shareholders to “decide to abandon” the funds subject to the rule.

The rule-making process was fraught with tensions within the S.E.C. and among government bodies. A new agency that Congress created after the financial crisis, the Financial Stability Oversight Council, proposed its own measures for money market funds and nudged the S.E.C. to finish its rules. But the influence of the council, which has special powers to lean on regulators, had rankled some at the S.E.C. The council is scheduled to discuss the new rules next week.

A version of this article appears in print on 07/24/2014, on page B3 of the NewYork edition with the headline: After Split Vote, S.E.C. Approves Rules on Money Market Funds.